A constant speed running apparatus for automobile performs automatic control to run an automobile at a set car speed. This constant-speed running apparatus for automobile is constituted principally with an actuator adjusting the degree of opening of a throttle valve, a controlling part and a driving part consisting of a micro computer which drives and controls this actuator and a running car speed detecting part, which adjusts the degree of opening of the throttle valve through the actuator, and adjusts fuel supplied to an engine, and controls the running car speed to become a set car speed. Such a conventional constant-speed running apparatus for an automobile usually includes an apparatus for preventing the controlling part from being put in an abnormal state of operation caused by static electricity or the like (watch dog timer. This device periodically sends a signal from the controlling part, and inputs this signal to a watch dog timer, and resets the micro-computer of the controlling part by the watch dog timer when the above-mentioned signal is not outputted for a predetermined time or more. The constant-speed running controlling state of the constant-speed running apparatus is cleared, and this thereby returns to the operation by the will of the driver.
However, there has been a problem that when only the portion transmitting a periodic signal of the controlling part is operated normally and an abnormal state takes place in the function performing a constant-speed running of the controlling part, the abnormal state cannot be detected by the above-mentioned constitution, and it is possible to fall into a state of dangerous operation.